My dealer just replaced my O2 sensor after the engine light was going on and off for a few weeks ("lean" codes). My complaint to the dealer going in was that the car idled roughly for the first couple of minutes when cold (and of course the fact that the engine light was going on and off). The cold idle roughness just came on over the last few weeks.

Now that the O2 sensor has been replaced, the idle is still rough for the first couple of minutes of a cold start (which I thought would be resolved with that fix).

The plugs look OK (45000 miles) and I am using only premium gas from a high-volume, brand-name gas station which I have used with no problem for years. There are no other OBD codes indicating another problem.

The car, as near as I can tell, runs fine when warmed up.

Does anyone have ideas what the problem could be? From past experience, I can't just turn it over to my dealer and expect a solution--I always have to do the research and make suggestions to him--or things just don't get fixed (he still charges me for things, but the problems remain).

 

Dirty IAC valve

 

Also try cleaning the throttle body and using Shell V-Power.....worked for me!

 

My battery failed within a couple of days of my post above. With a new battery, my problem has vanished. I suppose my rough idle when cold was due to low battery power on initial startup. It apparently smoothed out when the alternator had had a chance of bringing the battery's performance up a bit.

 

I have an 01 Cls, and I noticed that it idles kind of rough when it is cold, also.